This invention relates generally to construction implements and, more particularly, to trenchers which include a tractor that is propelled along terrain and a trenching implement adjustably mounted to the tractor. More particularly, the invention relates to a method and apparatus for controlling the trenching implement in order to trench to a target depth.
Underground utilities, such as gas lines, power lines, and communication cables, including coaxial cables and fiber-optic cables, are laid using a trencher that includes a tractor, which traverses the terrain on wheels or treads, and a trenching implement. The trenching implement is adjustably positioned at the rear of the tractor in the direction of movement of the tractor and is typically positioned with respect to the tractor by manual hydraulic controls manipulated by the operator. The operator is instructed to trench to a target depth, for example, 2 feet, or 26 inches or 32 inches, or whatever is desired, and to maintain the target depth irrespective of the terrain. Such trencher may additionally include a cable-feeding device, which feeds cable into the trench immediately behind the trenching implement and a pair of wings which pull the dirt, or spoil, back over the cable.
When the trencher tractor changes from a relatively planar terrain to one which slopes upwardly there is a tendency for the trenching implement to dig to a depth greater than the target depth. Likewise, when the trencher rounds the top of a hill, there is a tendency for the trenching implements to trench to a depth less than the target depth and may even come completely out of the ground. While experienced operators compensate for the non-level terrain, the operator is often distracted by other duties, such as guiding the tractor around telephone poles, fire hydrants, and other impediments. Therefore, it is not uncommon for the trenching implement to come completely out of the ground and to leave a portion of ground that is not trenched. Because the trencher may be concurrently laying underground cable, it is not possible for the operator to merely reverse the direction of the trencher and to retrench the same ground. Instead, the operator must stop the trencher and trench the missed terrain by hand. In addition to being difficult to operate, such trencher often produces unsatisfactory results with the actual depth of the trench varying from the target depth by great amounts. This makes locating of the underground cable difficult at a later date because the cable will not be at the depth location specified on the site map. Also, cables, such as fiber-optic cables, may be compromised if the trench takes an abrupt change of vertical direction, which would tend to put a kink in the cable. The operator must be exceptionally skillful to operate a manual trencher on uneven terrain without applying a kink to the cable.
Automatic controls have been proposed in order to maintain the actual depth of the trenching implement close to a target depth. However, such proposed automatic controls typically utilize a sensing device mounted to the trenching implement in order to sense a reference datum. A problem with such prior controls is that the sensor is exposed to the trenching operation and is vulnerable to soiling, damage, and even destruction. Furthermore, the use of a fixed reference datum limits the usefulness of such prior controls to relatively flat terrain which does not have large variations in elevation. Furthermore, the trencher is not capable of trenching to a constant depth perpendicular to the surface of the terrain.